In a photographic lens, focusing is generally performed by moving, relative to the film plane, the entire photographic lens or moving a part of the photographic lens. It is mechanically difficult to rapidly focus cameras having a large film size by moving the entire photographic lens because the entire photographic lens is large and heavy. Therefore, an inner-focus type construction is provided wherein the focus adjustment is performed by moving internal, relatively light, lens elements instead of moving the entire photographic lens.
Although the inner-focus type construction is well known in the field of imaging lenses for single-lens reflex cameras and video cameras, such a construction is rare in the case of cameras with a large film size because of the difficulty in favorably correcting aberrations in such cameras. Lens aberrations become more troublesome in proportion to the film size. Thus, even if the aberration amount is small enough not to be a problem for a lens with a short focal length (i.e., for a lens of a small-size film camera), blurring at the image surface is proportional to the focal length of the lens for a given amount of aberration, and thus may become readily noticeable for a lens with as large a focal length as is required by cameras having a large film size.
The present invention relates to an inner-focus type lens for use in photographic cameras, especially for such a lens having a long focal length so as to be suitable for use in cameras having a large film size. The object of the present invention is to provide a compact, inner-focus type lens having aberrations that are favorably corrected to such an extent so as to enable the photographic lens to be used in a camera having a large film size.